The present invention relates to multifunctional acrylate oligomers (Michael addition resins) formed from the reaction of acrylate monomers and oligomers (Michael acceptors) and β-dicarbonyl compounds (Michael donors) such as β-ketoesters, β-diketones, β-ketoamides and the like that can participate in the Michael addition reaction. In particular the invention relates to improved Michael addition resins with better cure response that have been modified to contain additional photoactive moieties, such as photoinitiators and/or photosensitizers. The improved Michael addition resins have multiple photoactive groups resulting directly from the Michael addition reaction as well as pendant Type I photoactive moieties (e.g., substituted benzoins, benzyl ketals, acetophenones or acyl phosphine oxides) or Type II photoactive moieties (e.g., substituted benzophenones, thioxanthones, camphorquinones, or bisimidazoles).
Multifunctional acrylates and methacrylates are commonly utilized in the preparation of crosslinked films, adhesives, foundry sand binders, composite materials, etc. U.S. Pat. No. 5,945,489 and U.S. Pat. No. 6,025,410 (both Ashland, Inc.) disclose liquid, uncrosslinked resins prepared via the Michael addition reaction of β-dicarbonyl compounds with multifunctional acrylates. The present invention comprises liquid, uncrosslinked, UV-curable Michael addition resins prepared utilizing β-dicarbonyl Michael donors and/or acrylate acceptors that are modified to contain pendant Type I (e.g., substituted benzoins, benzyl ketals, acetophenones or acyl phosphine oxides) or Type II (e.g., substituted benzophenones, thioxanthones, camphorquinones or bisimidazoles) photoactive moieties. The resulting oligomers thus possess either or both Type I and Type II photoactive functional groups that promote the addition polymerization of acrylic groups upon exposure to UV light. This structural change provides a supplementary chromophore for the initiation of free radical polymerization in addition to the photoactive substituted ketone formed during the Michael reaction.
The invention disclosed here demonstrates the advantageous use of these uncrosslinked resins alone or modified by reaction/blending with additional materials in coatings applications on a variety of substrates. These additional materials include a variety of acrylic and vinyl monomers and oligomers, primary and secondary amines, organonitro compounds, acid-functional materials, siloxanes, elastomers, waxes and others to modify and improve coatings performance.
The oligomers described above can be cured by all methods typically used to crosslink acrylic materials, though most advantageously by exposure to UV radiation. Cure, or crosslinking, is usually accomplished through a free radical chain mechanism, which may require any of a number of free radical-generating species such as peroxides, hydroperoxides, REDOX complexes, etc., which decompose to form radicals when heated, or at ambient temperature in the presence of amines and transition metal promoters. Electron beam (EB) radiation is another energy source suitable for initiating reaction of acrylic moieties.
The resins described in this invention offer significant advantages over traditional multifunctional acrylic monomers and oligomers in that they can be cured upon exposure to UV radiation without additional photoinitiator. Traditional multifunctional acrylates and/or oligomers will not cure upon exposure to UV radiation unless a photoinitiator, often at relatively high levels, is added to coating formulations. Traditional photoinitiators (e.g., benzophenone) can be insoluble, toxic, expensive, and contribute to film color, which can limit applicability of the coating over white and light-colored substrates. Furthermore, decomposition of some traditional photoinitiators results in cleavage products (e.g. benzaldehyde) that are of concern from a health perspective. Cleavage fragments may “bloom” to the surface of cured coatings where they can come into contact with skin. The invention disclosed here allows for the expeditious use of these traditional chromophore moieties while anchoring or “tethering” the largest fragments to the thermoset coating matrix.
The novel resins and blends disclosed herein exhibit performance properties that make them very effective coating materials. These properties can be modified greatly depending upon composition. Resins can be produced that show excellent adhesion to metals, plastics, wood, paper and glass. They exhibit wide ranges of hardness, toughness, flexibility, tensile strength, stain resistance, scratch resistance, impact resistance, solvent resistance, etc. Almost any desired coating performance parameter can be attained by proper selection of raw material building blocks.